Here, the RT value of each implant was lower compared with their respective IT values, consistent with other reports. The IT and RT values of 06S were highest, followed by 06D and 12S. In contrast, the differences between RT and IT values were highest for 12S, followed by 06D, and in 06S. We reported that the RT decreased more than IT. Thus, 06S had the lowest rate of decline (IT-RT) in the present study. Therefore, the single thread with a smaller pitch and helical angle is suitable for immediate loading. These inverse results may reflect the relation between lead angle or pitch length and the risk of slackening.

Thread design is a critical factor associated with primary implant stability. FEA loading studies show that relative vertical displacement is affected by thread pitch, torsion angle, and compactness. Under normal load, displacement was positively correlated with thread pitch and helix angle, and negatively with compactness. In low bone density jawbones, implant pitch, helix angle, and compactness have been reported to affect stability. Few studies have clarified the relationship between multiple threads and primary stability. A main advantage of an implant with multiple threads is quicker installation. However, this may be misunderstood by clinicians, because the double-threaded implant recommended by a manufacturer is used for all four types of immediate treatment loading. For example, our comparison of 12S to 06D led us to a different conclusion (i.e., better stabilities of 06D vs 12S and 06S vs 06D). When single- and double-threaded implants with the same pitch were compared, double-threaded implants were less stable because of greater damage to bone tissue damage, which is attributed to the high lead angle reported here as well as by others. Clinicians are advised to recognize the risk associated with using a multithreaded implant with a high lead angle, which may compromise primary stability because of greater bone tissue damage despite faster insertion.